Semiconductor laser producing visible light

ABSTRACT

A semiconductor laser producing visible light includes a first conductivity type semiconductor substrate, a first conductivity type AlGaInP cladding layer containing a first dopant impurity and disposed on the substrate, a semiconductor first spacer layer disposed on the cladding layer, an undoped InGaP active layer disposed on the first spacer layer wherein the first spacer layer inhibits intrusion of the first dopant impurity into the active layer, a semiconductor second spacer layer disposed on the active layer, a second conductivity type AlGaInP light guide layer containing a second dopant impurity and disposed on the active layer wherein the second spacer layer inhibits intrusion of the second dopant impurity into the active layer, a second conductivity type semiconductor current concentration and collection structure disposed on the light guide layer, and first and second electrodes disposed on the substrate and the current concentration and collection structure, respectively.

FIELD OF THE INVENTION

The present invention relates to a semiconductor laser producing lightvisible to the naked eye and, more particularly, to a semiconductorlaser producing visible light and having a long lifetime.

BACKGROUND OF THE INVENTION

FIG. 2 is a cross-sectional view showing the structure of a prior artsemiconductor laser producing visible light, i.e., light visible to thenaked eye. The laser includes an n-type gallium arsenide (GaAs)substrate 1 on which numerous layers are successively disposed. Ann-type aluminum gallium indium phosphide (AlGaInP) cladding layer 2 isdisposed on the substrate 1, an undoped indium gallium phosphide (InGaP)active layer 4 is disposed on the cladding layer 2, a p-type AlGaInPlight guide layer 5 is disposed on the active layer 4, and a p-typeInGaP etch stopping layer 6 is disposed on the light guide layer 5.Those layers 2, 4, 5, and 6 are all successively grown on the substrate1 by conventional techniques, for example, by metal organic chemicalvapor deposition (MOCVD). A current blocking layer 12 and mesa structure13 are disposed on the etch stopping layer 6 to concentrate the currentflow in the central portion of the active layer 4 and to form a lossguide structure for transverse oscillation mode stabilization.

To form the mesa structure and current blocking layer, initially ap-type AlGaInP cladding layer 7, a p-type InGaP transition layer 8, anda p-type GaAs contacting layer 9a are successively grown on the etchstopping layer 6. Thereafter, an etching mask is formed on part of thecontacting layer 9a and the mesa 13 is formed by etching the unmaskedportions of layers 7, 8, and 9a. An etchant that etches AlGaInP far morerapidly than InGaP is employed. Thus, when the etch stopping layer 6 isexposed, the rate of etching declines rapidly and damage to theunderlying light guide layer 5 that would occur if the etch stoppinglayer 6 were absent is avoided.

The current blocking layer 12 of n-type GaAs is grown on the etchstopping layer 6 at both sides of the mesa 13. By using MOCVD or anotherselective growth process with the etching mask still in place on theremaining contacting layer 9a, the current blocking layer 12 does notgrow on top of the mesa 13. Finally, a second p-type contacting layer 9bis grown on the top of the mesa 13 and on the current blocking layer 12after the etching mask is removed. Electrodes 10 and 11 are formed onthe substrate 1 and the second contacting layer 9b, respectively, tocomplete the semiconductor laser. Either before or after the electrodesare formed, the structure is cleaved to form the opposed facets of thelaser.

The prior art laser structure is described above without specificationof the relative concentrations of the various elements in the ternaryand quaternary alloys. The undoped active layer 4 in a semiconductorlaser producing visible light typically is usually approximately Ga₀.5In₀.5 P. The light guide layer 5 and the cladding layers 2 and 7 areusually approximately Al₀.28 Ga₀.23 In₀.49 P. Likewise, in the structureof FIG. 2, the transition layer 8 is intended to have an energy band gapintermediate those of the layers 7 and 9a to reduce the voltage dropthat occurs when the GaAs contacting layer directly contacts the AlGaInPcladding layer. In this laser structure, transverse mode oscillation isstabilized by a loss guide structure. Light produced in the active layer4 that reaches the current blocking layer 12 is absorbed because of thesmaller energy band gap of GaAs. Within the mesa 13, the light is notabsorbed because of the larger energy band gap of the AlGaInP claddinglayer 7. The loss guide structure concentrates light at the mesa 13,stabilizing the oscillation mode of the laser.

The conductivity types of the layers of the laser of FIG. 2 aredetermined during growth by including appropriate dopant impurities inthe growing layers. For example, the dopant impurity that typicallyprovides p-type conductivity in the light guide layer 5 and the claddinglayer 7 is zinc (Zn). The dopant impurity used to produce n-typeconductivity in the cladding layer 2 is typically selenium (Se) orsilicon (Si). When Zn is employed as a p-type dopant impurity inAlGaInP, e.g., in the light guide layer 5 and the cladding layer 7, ithas been observed to have a relatively low degree of electricalactivity. In other words, a relatively small proportion, for example,only forty percent, of the incorporated Zn atoms are ionized and act asacceptors. The remainder of the Zn does not affect the electricalproperties of the laser. To compensate for that low degree ofionization, a relatively large amount of Zn is incorporated into thegrowing layers. The diffusion coefficient of Zn in AlGaInP is largerthan that in GaInP. As a result of a relatively high concentration of Znand the relative diffusion coefficients of Zn in the cladding and activelayers, unusual dopant impurity concentrations can occur in the laserstructure. Those dopant impurity concentration abnormalities areaccentuated by a known interaction between Zn and Se.

The unusual dopant impurity concentrations that can occur in the laserstructure of FIG. 2 are illustrated in FIG. 3. There, the relativeconcentrations of Zn and Se in the cladding layer 2, the active layer 4,and the light guide layer 5 are plotted as a function of position. Theexpected concentration of Zn in the layer 4, absent the highconcentration of Zn in the light guide layer 5 and the differentdiffusion constants of the layers 2 and 7, is illustrated by a brokenline. Because of the difference in diffusion coefficients of Zn in theactive layer 4 and the light guide layer 5, an abnormally largeconcentration of Zn can occur near the interface of the active layer 4and the light guide layer 5, as shown in FIG. 3. The increased Znconcentration and the elevated temperatures employed in growing thevarious layers of the laser structure cause the Zn to diffuse into theactive layer 4 that is desirably neither n-type nor p-type. Theabnormally high Zn concentration at the interface effectively provides ahigh concentration diffusion source, accelerating the intrusion of Zninto the active layer 4. The Zn diffusion can occur during growth of thelayers of the semiconductor laser, during other high temperature processsteps in the fabrication of the laser, or during operation at elevatedtemperatures. When the concentration of Zn in the active layer 4increases, undesired charge carrier recombination occurs in that layer,reducing the light output of the semiconductor laser. To compensate forthe reduced light output, the current flowing through the laser may beincreased, increasing the operating temperature of the laser,accelerating further Zn diffusion and premature failure, i.e., shortenedlifetime, of the laser.

SUMMARY OF THE INVENTION

The present invention is directed to providing a semiconductor laserproducing visible light of acceptable intensity from initial operationand during a long lifetime.

An object of the present invention is prevent dopant impurities in asemiconductor laser structure producing invisible light from prematureintrusion into the active layer of the semiconductor laser and therebycausing a reduction in light output.

According to a first aspect of the invention, a semiconductor laserproducing visible light includes a first conductivity type semiconductorsubstrate, a first conductivity type AlGaInP cladding layer containing afirst dopant impurity and disposed on the substrate, a semiconductorspacer layer disposed on the cladding layer, an undoped InGaP activelayer disposed on the spacer layer wherein the spacer layer inhibitsintrusion of the first dopant impurity into the active layer, a secondconductivity type AlGaInP light guide layer disposed on the activelayer, a second conductivity type semiconductor current concentrationand collection structure disposed on the light guide layer, and firstand second electrodes disposed on the substrate and the currentconcentration and collection structure, respectively.

In another aspect of the invention, a semiconductor laser producingvisible light includes a first conductivity type semiconductorsubstrate, a first conductivity type AlGaInP cladding layer disposed onthe substrate, an undoped InGaP active layer disposed on the firstconductivity type cladding layer, a semiconductor spacer layer disposedon the active layer, a second conductivity type AlGaInP light guidelayer containing a first dopant impurity and disposed on the spacerlayer wherein the spacer layer inhibits intrusion of the first dopantimpurity into the active layer, a second conductivity type semiconductorcurrent concentration and collection structure disposed on the lightguide layer, and first and second electrodes disposed on the substrateand the current concentration and collection structure, respectively.

According to a third aspect of the invention, a semiconductor laserproducing visible light includes a first conductivity type semiconductorsubstrate, a first conductivity type AlGaInP cladding layer containing afirst dopant impurity and disposed on the substrate, a semiconductorfirst spacer layer disposed on the cladding layer, an undoped InGaPactive layer disposed on the first spacer layer wherein the first spacerlayer inhibits intrusion of the first dopant impurity into the activelayer, a semiconductor second spacer layer disposed on the active layer,a second conductivity type AlGaInP light guide layer containing a seconddopant impurity and disposed on the active layer wherein the secondspacer layer inhibits intrusion of the second dopant impurity into theactive layer, a second conductivity type semiconductor currentconcentration and collection structure disposed on the light guidelayer, and first and second electrodes disposed on the substrate and thecurrent concentration and collection structure, respectively.

Other objects and advantages of the present invention will becomeapparent from the detailed description that follows. The detaileddescription and specific embodiments of the invention are provided forillustration only, since various additions and modifications within thespirit and scope of the invention will be apparent to those of skill inthe art from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are a cross-sectional view and a perspective view,respectively, of a semiconductor laser producing visible light inaccordance with an embodiment of the invention.

FIG. 2 is a cross-sectional view of a semiconductor laser producingvisible light according to the prior art.

FIG. 3 is a graph of relative dopant impurity concentrations in theactive layer and the layers adjacent the active layer in the prior artlaser structure of FIG. 2.

FIGS. 4(a) and 4(b) are graphs of the photo-luminescence spectra of theactive layers of a prior art laser and of a laser in accordance to theinvention, respectively.

FIG. 5 is a graph showing light output as a function of the currentdensity in a prior art laser and in a laser in accordance with theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1(a) and 1(b) show cross-sectional and perspective views,respectively, of a semiconductor laser producing visible light inaccordance with an embodiment of the invention. In these figures, thesame reference numbers designate the same elements already describedwith respect to FIG. 2. The elements already described do not need to bedescribed again. In addition to the elements of the prior artsemiconductor laser structure of FIG. 2, the semiconductor laserstructure of FIG. 1(a) includes a semiconductor first spacer layer 3adisposed between the n-type cladding layer 2 and the active layer 4 anda semiconductor second spacer 3b disposed between the active layer 4 andthe light guide layer 5. The first and second spacer layers 3a and 3bare AlGaInP and, preferably, are undoped.

The structure of FIG. 1(a) is prepared in the same manner as thestructure of FIG. 2. However, after the growth of the cladding layer 2,the spacer layer 3a is grown before the active layer 4 is grown.Likewise, after the growth of the active layer 4 and before the growthof the light guide layer 5, the second spacer layer 3b is grown.Otherwise, the structure of FIG. 1(a) can be produced in the same way asthe structure of FIG. 2 including cleaving to produce the opposed facets14 and 15, shown in FIG. 1(b), transverse to the layers 2 through 11.The facets 14 and 15, with the active layer 4, the cladding layers 2 and7, and the other layers between the cladding layers 2 and 6, form aresonant optical cavity that supports laser oscillation. The mesa 13 maybe formed by etching the contacting layer 9a with equal volumes oftartaric acid and hydrogen peroxide, the InGaP transition layer 8 withequal volumes of hydrochloric and phosphoric acids, and the AlGaInPcladding layer with equal volumes of sulfuric acid and water. Typically,the first and second spacer layers 3a and 3b have the same compositionswith respect to the proportions of Al, Ga, and In as the cladding andlight guide layers 2 and 5. The layers employed in the laser structuretypically have the following thicknesses:

                  TABLE 1                                                         ______________________________________                                        Layer               Thickness                                                 ______________________________________                                        claddng layer 2     1 micron                                                  spacer layer 3a     0.05 to 0.1 micron                                        active layer 4      0.05 to 0.1 micron                                        spacer layer 3b     0.05 to 0.1 micron                                        light guide layer 5 0.3 micron                                                etch stopping layer 6                                                                             0.005 to 0.01 micron                                      cladding layer 7    0.7 micron                                                transition layer 8  0.1 micron                                                contacting layer 9a 0.1 to 0.4 micron                                         contacting layer 9b 3 microns.                                                ______________________________________                                    

In the embodiment of the invention shown in FIG. 1(a), if Zn atomsdiffuse from the light guide layer 5 toward the active layer 4, theymust first pass through the spacer layer 3b. While spacer layer 3b isquite thin, it is preferably not intentionally doped and, thereby, actsas a sink for the diffusing Zn atoms, preventing the Zn atoms fromreaching the active layer 4. Likewise, the dopant impurity, such as Seand Si, producing the n-type conductivity in the cladding layer 2 isprevented from reaching the active layer 4 by the preferably undopedfirst spacer layer 3a. Together, the spacer layers 3a and 3b prevent ordelay the recognized interaction between Zn and Se atoms, therebymaintaining the active layer 4 as an undoped layer for a relatively longperiod of time. The delay in the intrusion of dopant impurities into theactive layer 4 extends the lifetime of the laser by preventing apremature decrease in the light output for a particular current densityflowing through the laser. Because the intrusion of dopant impurities isprevented both initially and during operation of the laser, the lifetimeof the laser is extended as compared to the prior art structure.

In order to test the effectiveness of the spacer layers in theinvention, the photoluminescence spectra of active layers of laserdiodes according to the invention and of laser diodes according to theprior art were measured. Laser structures were fabricated according tothe prior art and according to the invention and the overlying layers,including a spacer layer, were removed to expose the active layers. Theactive layers thus exposed were subjected to incident light and thespectrum of the resulting luminescence measured. Examples of themeasured spectra are shown in FIG. 4(a) for the prior art laserstructure and in FIG. 4(b) for the laser structure according to theinvention. In FIG. 4(a), two emission peaks labeled A and B wereobserved. The peak A is attributable to the inherent characteristics ofthe active layer whereas the peak B is attributable to transitions to aZn dopant energy level, i.e., to Zn atoms, that are present within theactive layer of the prior art structure. By contrast, in the structureaccording to the embodiment of the invention, only the luminescence peakA is observed, confirming that Zn has not diffused into the active layer4 during fabrication of the laser structure.

In addition to the advantage of extended lifetime achieved in theinvention, a laser according to the invention has a reduced thresholdcurrent density for producing laser oscillation as well as increasedefficiency, i.e., light output, for a particular current density. InFIG. 5, the measured light output as a function of current density isplotted both for a semiconductor laser according to the prior art and asemiconductor laser according to the invention. Relative light output isplotted on the ordinate and the current density for the respective laserstructures is plotted on the abscissa. As shown in FIG. 5, the thresholdcurrent density J_(th) for the laser according to the invention is aboutthirty percent less than the current density for the laser structureaccording to the prior art. In addition, a relatively intense lightoutput is produced by the laser according to the invention before laseroscillation even begins in the laser according to the prior art.

In the specific embodiment of the invention described with respect toFIG. 1(a), the spacer layers 3a and 3b are AlGaInP. However, othermaterials, such as AlGaAs, may be employed as the spacer layers.Preferably, the spacer layers have a lower index of refraction and ahigher energy band gap than the active layer. For an active layer of theapproximate composition Ga₀.5 In₀.5 P, a spacer layer Al_(x) Ga_(1-x) Asshould have x>0.6. The spacer layers are preferably very thin, e.g.,0.05 to 0.1 micron, and, therefore, do not adversely affect theelectrical properties of the laser while effectively inhibiting theintrusion of dopant impurities into the active layer.

Although the embodiment of the invention shown in FIG. 1(a) includes twospacer layers 3a and 3b, each spacer layer contributes to the advantagesachieved in the invention. The prevention of dopant impurity diffusioninto the active layer can be at least partially achieved even if onlyone of the spacer layers 3a and 3b is present in the structure. Theinteraction between Se and Zn produces enhanced adverse effects whenboth dopant impurities reach the active layer 4. Thus, the presence ofonly one of the spacer layers retarding the intrusion of either Zn or Seis effective to extend the lifetime and reduce the threshold current ofa semiconductor laser. An interaction similar to that of Se and Zn butsmaller in effect exists for Si and Zn. Therefore, the advantages of theinvention are achieved with only one of the spacer layers presentregardless of the dopant producing n-type conductivity in the claddinglayer 2.

The structure of FIG. 1(a) is described with certain layers of p-typeconductivity and other layers of n-type conductivity. However, therespective conductivities of the various layers can be reversed whileachieving the advantages of the invention since it is the exclusion ofdopant impurities from the active layer that is desired without regardto the layer that is the source of the dopant impurities.

The embodiment of the invention shown in FIG. 1(a) employs a currentconcentration and collection structure having a forward mesa includinglayers 7, 8, 9a, and 9b of p-type conductivity and the n-type currentblocking layer 12. However, the invention is equally applicable to laserstructures employing different current concentration and collectionstructures. Those structures concentrate the current flow in the centralportion of the active layer for laser oscillation and currentcollection. Examples of such structures are forward and reverse mesastructures, i.e., structures in which the side surfaces converge anddiverge in the direction of the active layer, respectively, and stripegroove structures. The type of mesa, i.e., forward and reverse, can bealtered merely by changing the orientation of the etching mask relativeto the crystallographic orientation of the substrate. In stripe groovestructures, the current blocking layer is grown before the secondcladding layer is grown. The current blocking layer is etched to exposethe underlying active layer or a light guide layer. Therefore, thesecond cladding layer is grown on the current blocking layer and exposedunderlying layer. In summary, the particular cross-sectional shape ofthe current concentration and collection structure of the laser disposedon the side of the active layer 4 opposite the substrate 1 is notcritical to the novel structure nor the advantages provided by the novelstructure.

I claim:
 1. A semiconductor laser producing visible light comprising:afirst conductivity type semiconductor substrate; a first conductivitytype AlGaInP cladding layer containing a first dopant impurity disposedon said substrate; a semiconductor spacer layer disposed on saidcladding layer; an undoped InGaP active layer disposed on said spacerlayer wherein said spacer layer inhibits intrusion of said first dopantimpurity into said active layer; a second conductivity type AlGaInPlight guide layer disposed on said active layer; a second conductivitytype semiconductor current concentration and collection structuredisposed on said light guide layer; first and second opposed facetstransverse to said cladding layer, said spacer layer, said active layer,and said light guide layer forming a resonant optical cavity forsupporting laser oscillation; and first and second electrodes disposedon said substrate and said current concentration and collectionstructure, respectively.
 2. The semiconductor laser of claim 1 whereinsaid spacer layer is undoped.
 3. The semiconductor laser of claim 2wherein said spacer layer is AlGaInP.
 4. The semiconductor laser ofclaim 2 wherein said spacer layer is Al_(x) Ga_(1-x) As.
 5. Thesemiconductor laser of claim 4 wherein x>0.6.
 6. The semiconductor laserof claim 1 wherein said first dopant impurity is selected from the groupconsisting of Se and Si.
 7. The semiconductor laser of claim 1 whereinsaid light guide layer contains a second dopant impurity.
 8. Thesemiconductor laser of claim 7 including a semiconductor second spacerlayer disposed between said active layer and said light guide layerinhibiting intrusion of said second dopant impurity into said
 9. Thesemiconductor laser of claim 8 wherein said second dopant impurity isZn.
 10. The semiconductor laser of claim 1 including an InGaP etchstopping layer disposed between said light guide layer and said currentconcentration and collection structure.
 11. The semiconductor laser ofclaim 1 wherein said current concentration and collection structurecomprises an AlGaInP second cladding layer disposed on said light guidelayer and a second conductivity type GaAs contacting layer disposedbetween said second cladding layer and said second electrode.
 12. Thesemiconductor laser of claim 11 wherein said current concentration andcollection structure comprises an InGaP transition layer disposedbetween said second cladding layer and said contacting layer.
 13. Asemiconductor laser producing visible light comprising:a firstconductivity type semiconductor substrate; a first conductivity typeAlGaInP cladding layer disposed on said substrate; an undoped InGaPactive layer disposed on said first conductivity type cladding layer; asemiconductor spacer layer disposed on said active layer; a secondconductivity type AlGaInP light guide layer containing a first dopantimpurity and disposed on said spacer layer wherein said spacer layerinhibits intrusion of said first dopant impurity into said active layer;a second conductivity type semiconductor current concentration andcollection structure disposed on said light guide layer; first andsecond opposed facets transverse to said cladding layer, said activelayer, said spacer layer, and said light guide layer forming a resonantoptical cavity for supporting laser oscillation; and first and secondelectrodes disposed on said substrate and said current concentration andcollection structure, respectively.
 14. The semiconductor laser of claim13 wherein said spacer layer is undoped.
 15. The semiconductor laser ofclaim 14 wherein said spacer layer is AlGaInP.
 16. The semiconductorlaser of claim 14 wherein said spacer layer is Al_(x) Ga_(1-x) As. 17.The semiconductor laser of claim 16 wherein x>0.6.
 18. The semiconductorlaser of claim 13 wherein said first dopant impurity is Zn.
 19. Thesemiconductor laser of claim 13 wherein said first conductivity typecladding layer contains a second dopant impurity.
 20. The semiconductorlaser of claim 19 wherein said second dopant impurity is selected fromthe group consisting of Se and Si.
 21. The semiconductor laser of claim13 including an InGaP etch stopping layer disposed between said lightguide layer and said current concentration and collection structure. 22.The semiconductor laser of claim 13 wherein said current concentrationand collection structure comprises an AlGaInP second cladding layerdisposed on said light guide layer and a second conductivity type GaAscontacting layer disposed between said second cladding layer and saidsecond electrode.
 23. The semiconductor laser of claim 22 wherein saidcurrent concentration and collection structure comprises an InGaPtransition layer disposed between said second cladding layer and saidcontacting layer.